The influences of iron (III) and lead (II) contaminants on lime-stabilised clay

This paper represents a continuation of the work published by Boardman et al., who concluded that the mechanisms of lime stabilisation were distinctly different for two clays of different mineralogy: English china clay, for which short-term flocculation is dependent on pH and long-term strength is dominated by the production of calcium aluminate hydrate (CAH); and Wyoming bentonite, for which strength is dominated by short-term flocculation that is only enhanced by high pH, and long-term strength is a function of the production of calcium silicate hydrate (CSH). To investigate further these variations in the lime stabilisation mechanisms, and to assess the effects of contamination on the stabilisation and solidification process, the studies were repeated using the two clay minerals contaminated with 5000 ppm of either Pb²⁺ or Fe³⁺ ions prior to the addition of lime. A similar methodology was adopted as in the previous work, in which the time-dependent physico-chemical mechanisms of reactions were studied by comparing the results from chemical batch leaching tests with the undrained shear strengths and Atterberg limits after three different curing periods. The study concluded that contaminants had a large effect on the initiation and development of the lime–clay reactions both during short-term modification and during the longer-term solidification. However, the effects were found to be clay mineral dependent. It was not possible simply to conclude on the general effects of a particular contaminant, its potential effects having to be considered in relation to the dominant clay mineral. The most profound effects were observed in the mineral with pH-dependent charge (typical of clays commonly found in the UK), where both flocculation and mineral dissolution were substantially altered.